Run-on Sentences and Comma Splices

A run-on sentence has two independent clauses, as the following.

Jim prepared well for the examination he passed in it.

Here, the sentence has actually two independent sentences and there is no punctuation or conjunction to separate them. This is an abject use of language. In British and American English, this kind of writing is a grammatical error, and should be avoided.

To avoid the error, you may use a comma in the middle of two sentences, as in:

Jim prepared well for the examination, he passed in it.

But this sort of sentence is known as a comma splice. This is also an indefensible mistake. You should never use two independent sentences in a single sentence with a comma or without any punctuation. To make it correct, the only way is adding a conjunction like 'and' or 'but' thus making one of the clauses dependent on the other.

Jim prepared well for the examination, and he passed in it.

When separating two independent clauses in a sentence, the correct punctuation to use is always a semicolon.

Jim prepared well for the examination; he passed in it.

So, there are three ways you can correct a run-on sentence or a comma splice.

1. By adding a conjunction like 'and or 'but.'

2. By replacing a comma in a comma splice with a semicolon.

3. By writing the independent clauses as independent sentences separated by the period.

Though this is the case, in poetry, comma splices may be used and are generally accepted as correct. Also, in very simple small sentences, people can go well without the semicolon.

Jack ran, Jill followed.

In writing, run-on sentences and comma splices are a very ridiculous construction. You should analyze your commas well to avoid it. Also, overusing commas is not a good practice in writing. You should check out the comma rules of punctuation.